Engine emissions testing has become increasing important as legislative pollution controls are enacted and public awareness of environmental concerns heightens. Typical test procedures involve acquiring a sample of the exhaust and then analyzing the sample for its constituents. The testing may be "real-time" such that the sample is analyzed as it is collected, or it may be collected remotely in time or location from the analysis.
A problem with many existing sampling techniques is that they rely on a laboratory setting and relatively large equipment to provide a sample whose quantity reflects changing engine speed and load and hence changing amounts of exhaust flow during the sample period. They do not permit the engine to be tested "in the field" under real-life operating conditions.